1. Field of the Invention
The present invention relates to a device for frequency stabilizing an RF excitation laser comprising a resonant cavity filled with a laser gas and having an impedance, a radio-frequency (RF) alternating power generator having an output impedance, electrodes for exciting the laser gas connected to the generator and means adapted for varying the frequency of the laser.
2. Description of the Prior Art
It will be recalled that a laser has multiple applications, not only medical but also military applications for example.
The emission or absorption spectral lines in the gain profiles of lasers, i.e. in the curves representative of the light intensity of the lasers as a function of the frequency, about particular resonance frequencies, have been used for a long time for stabilizing the lasers, i.e. for fixing the emission frequency on one of the spectral lines, But for that, a light intensity detector was required, and that was not practical.
Then, since the efficient excitation sections of the molecules of the laser gas media, by electron collision, vary depending on their nature and, for given molecules, depending on their energy level, it was realized that, about resonance frequencies, the population of the levels participating in the amplification or the absorption varies in the same manner as the light intensity as a function of the frequency of the incident optical emission. Now, variation of population causes variation in the macroscopic ionization properties of the laser plasma and so of its impedance.
Since the impedance of a laser plasma varies then with the frequency as the light intensity, it was then proposed to stabilize lasers by means of their impedance. However, such an optogalvanic stabilization has only been practised up to now for continuous excitation lasers and more particularly for stabilizing them at the top of a spectral line. Attempts have been made to apply this optogalvanic stabilization technique to non continuous excitation lasers but unsuccessfully. It should be mentioned that in the case of continuous excitation lasers application of the technique was simple because of the different natures of the excitation signal and of the stabilization signal, one being continuous and the other alternating.
The applicant began to study again the problem of optogalvanic stabilization of non continuous excitation lasers, with RF excitation, after the appearance in 1980-1981 of optogalvanic RF discharge spectroscopy. It is a matter of producing, in the gas medium of a cell to be examined, atoms excited by a plasma caused by a radiofrequency oscillator, coupled to the cell, either capacitively by external electrodes fixed to the cell or inductively by means of a coil surrounding the cell. Because of the coupling, the impedance of the gas medium of the cell forms one of the elements of the load of the oscillator. This impedance is modified by photon absorption caused by incident laser radiation at variable frequency, and it is this impedance modification which is detected, either directly at the level of the oscillator whose frequency or amplitude is modulated accordingly or by means for example of an antenna. This laser spectroscopy technique is more especially described in the article by D. R. Lyons et al. Published in Optics Communications, volume 38, No. 1, of 1.7.1981 and in the article by C. Stanciulescu et al. published in Applied Physics Letters 37(10) of 15.11.1980.
However, the analysis of a gas by optogalvanic RF discharge laser spectroscopy is one thing and the optogalvanic stabilization of a laser cavity is another, if only because of the fact that, in the problem raised, the cell containing the gas medium and the laser cavity are intermingled, and that, in known spectroscopy techniques, the RF powers brought into play are particularly small.
However it may be, the Applicant then resolved the problem of the optogalvanic stabilization of an RF excitation laser and today proposes his invention.